Electronics, microelectronics and microelectromechanics require as starting materials semiconductor wafers with extreme requirements made of global and local flatness, single-side-referenced flatness (nanotopology), roughness and cleanness. Semiconductor wafers are wafers composed of semiconductor materials such as elemental semiconductors (silicon, germanium), compound semiconductors (for example composed of an element of the third main group of the periodic table such as aluminum, gallium or indium and an element of the fifth main group of the periodic table such as nitrogen, phosphorus or arsenic) or the compounds thereof (for example Si1-xGex, 0≦x≦1).
Semiconductor wafers are typically produced by means of a multiplicity of successive process steps which can generally be classified into the following groups:
(a) producing a usually monocrystalline semiconductor rod;
(b) slicing the rod into individual wafers;
(c) mechanical processing;
(d) chemical processing;
(e) chemomechanical processing;
(f) if appropriate producing layer structures.
In the production of semiconductor wafers for particularly demanding applications, advantageous sequences in this case include sequences which comprise at least one processing method in which both sides of the semiconductor wafers are simultaneously processed in material-removing fashion in one processing step by means of two working surfaces, to be precise in such a way that the processing forces acting on the semiconductor wafer on the front and rear sides during the material removal substantially compensate for one another and no constraining forces are exerted on the semiconductor wafer by a guide apparatus, that is to say that the semiconductor wafer is processed in “free floating” fashion.
In the prior art, preference is given to sequences in which both sides of at least three semiconductor wafers are simultaneously processed in material-removing fashion between two ring-shaped working disks, wherein the semiconductor wafers are inserted loosely into receiving openings of at least three guide cages (carriers) toothed on the outside, which are guided by means of a rolling apparatus and the outer toothing under pressure on cycloidal paths through the working gap formed between the working disks, such that in this case they can rotate completely around the midpoint of the double-side processing apparatus. Methods that employ rotating carriers and process both sides of a plurality of semiconductor wafers simultaneously in material-removing fashion over the whole area in this way include double-side lapping (“lapping”), double-side polishing (DSP) and double-side grinding with planetary kinematics (“planetary pad grinding”, PPG). Of these, in particular DSP and PPG are of particular importance. In contrast to lapping, the working disks in the case of DSP and in the case of PPG additionally each comprise a working layer, the mutually facing sides of which constitute the working surfaces. PPG and DSP are known in the prior art and will be described briefly below.
“Planetary pad grinding” (PPG) is a method from the group of mechanical processing steps which brings about a material removal by means of grinding. It is described for example in DE102007013058A1, and an apparatus suitable therefor is described for example in DE19937784A1. In the case of PPG, each working disk comprises a working layer containing bonded abrasive. The working layers are present in the form of structured grinding pads which are fixed on the working layers adhesively, magnetically, in a positively locking manner (for example hook and loop fastener) or by means of vacuum. The working layers have a sufficient adhesion on the working disk in order not to be displaced, deformed (formation of a bead) or detached during processing. However, they can easily be removed from the working disks by means of a peeling movement and can therefore rapidly be exchanged, such that, without long set-up times, it is possible to change rapidly between different types of grinding pad for different applications. Suitable working layers in the form of grinding pads designed to be self-adhesive on the rear side are described for example in U.S. Pat. No. 5,958,794. The abrasive used in the grinding pads is preferably diamond.
Double-side polishing (DSP) is a method from the group of chemomechanical processing steps. DSP processing of silicon wafers is described for example in US2003/054650A and an apparatus suitable therefor is described in DE10007390A1. In this description, “chemical mechanical polishing” should be understood exclusively to mean a material removal by means of a mixed effect, comprising chemical etching by means of an alkaline solution and mechanical erosion by means of loose grain dispersed in the aqueous medium, which is brought into contact with the semiconductor wafer by a polishing pad, which contains no hard substances that come into contact with the semiconductor wafer, and thus brings about a material removal from the semiconductor wafer under pressure and relative movement. In the case of DSP, the working layers are present in the form of polishing pads, and the latter are fixed on the working disks adhesively, magnetically, in a positively locking manner (for example hook and loop fastener) or by means of vacuum. The alkaline solution preferably has a pH value of between 9 and 12 during chemical mechanical polishing, and the grain dispersed therein is preferably a colloidally disperse silica sol having grain sizes of the sol particles of between 5 nm and a few micrometers.
What is common to PPG and DSP is that the flatness and parallelism of the working surfaces directly determine the obtainable flatness and parallelism of the semiconductor wafer processed by them. For PPG this is described in DE DE102007013058A1. For particularly demanding applications, particularly stringent requirements made of the plane-parallelism of the semiconductor wafer and thus of the plane-parallelism of the working surfaces are applicable.
The flatness of the working surface is firstly critically determined by the flatness of the working disk which carries the working layer. The following methods are known for making the working disks of double-side processing apparatuses as flat as possible:
By way of example, turning of the working disk blank by means of chip removal by a turning tool is known. The face turning is preferably effected after the working disk has been mounted in the double-side processing apparatus, since subsequent mounting can strain or deform the working disk again. Alternatively, the working disk can also be processed prior to mounting on a correspondingly larger processing apparatus for example by lapping toward planarity and then has to be mounted in a manner exhibiting particularly low strain. What is common to all of the known measures, however, is that they can admittedly improve the flatness of the working disk, but not to the extent that would be necessary for the production of semiconductor wafers for particularly demanding applications.
The parallelism of the working surfaces with respect to one another is likewise firstly critically determined by the parallelism of the working disks each carrying a working layer. The following methods are known for making the working disks of double-side processing methods as parallel as possible to one another:
Firstly, one working disk, preferably the lower one, which is generally mounted rigidly in the double-side processing apparatus, is made as flat as possible by turning after incorporation or by lapping on a separate processing apparatus before incorporation into the double-side processing apparatus. Then, the other working disk, preferably the upper one, which is generally mounted cardanically and can thereby at least globally on average always be oriented parallel to the lower working disk, is incorporated into the double-side processing apparatus and lapped in against the lower working disk. Preceding face turning of the upper working disk in a separate processing apparatus is conceivable; however, in that case, it is necessary, finally, for the two working disks, after incorporation into the double-side processing apparatus, to be lapped against one another in order to remove the processing traces of turning or the offsets from the multiple changing or redressing of the turning tool that is necessary owing to the large chipping volume.
Since the working disks finally always have to be lapped, at the end of the leveling process they have a convex profile and their surfaces facing one another therefore run parallel to one another only to an insufficient extent.
The prior art discloses possibilities for ensuring that a best possible plane-parallelism of the working surfaces—once it has been established—is maintained even under thermal and mechanical cyclic loading. A particularly stiff working disk with good cooling is described for example in DE10007390A1. Possibilities for actively setting the working disk form are disclosed for example in DE102004040429A1 or DE102006037490A1. However, these methods for the targeted deformation of the working disks during processing are unsuitable for making an initially uneven working disk flat to an extent such that the working surface of a working layer applied on the working disk has the flatness and parallelism of both working surfaces with respect to one another as required for the production of semiconductor wafers for particularly demanding applications.
Finally, the flatness of the working surfaces and the parallelism of both working surfaces with respect to one another are determined by the thickness profile of the working layers applied to the working disks. The working layer can, if it is highly constant in its thickness and elastic, at best simulate the form of the working disk.
Finally, the prior art discloses methods for trimming the working layer. Trimming is understood to mean the targeted material removal from a tool. A distinction is made between shaping trimming (“truing”) and trimming that alters the surface properties of the tool (“dressing”, “conditioning”, “seasoning”). In the case of shaping trimming, material is removed from the tool with the aid of suitable trimming apparatuses in such a way that a desired target form of the elements of the tool which come into contact with the workpieces arises. In contrast thereto, in the case of trimming that only alters the surface properties of the tool, so little material is removed that the desired property change, for example roughening, cleaning or redressing, is just achieved, but a critical change in the form of the tool is avoided in the process.
In the case of DSP, however, shaping trimming of the working layers (polishing pads) cannot be carried out since the useful layer of a polishing pad is extremely thin. The useful layer is so thin because the polishing pad is subject to practically no material-removing wear in the course of its use. Since shaping trimming cannot be carried out in the case of DSP, an unevenness of the working surface resulting from an uneven working disk cannot be corrected.
In the case of PPG, the working layer (grinding pad), by means of the abrasive bonded in it, enters into engagement with the semiconductor wafer and brings about the material removal under pressure and with relative movement. The grinding pad is therefore subject to wear. Since the PPG grinding pad is subject to wear, its useful layer generally has a considerable thickness (at least a few tenths of a millimeter), and so economic use without frequent production interruptions caused by changing the grinding pad is possible and its flatness can be reestablished by repeated trimming. In the prior art, directly after a new grinding pad has been applied, trimming is carried out in order to expose abrasive grain at the working surface (initial dressing). One method for initial dressing is described for example in T. Fletcher et al., Optifab, Rochester, N.Y., May 2, 2005.
Both initial dressing by itself and regular trimming for reestablishing the form of the working surface are associated with such small material removals from the working layer that this does not significantly shorten the service life of the grinding pad.
In principle, in the case of PPG, in contrast to DSP, it is possible to trim the working layer by means of considerably lengthened shaping trimming such that a flat working surface is obtained even on an uneven working disk such as cannot be produced better in the prior art. In this case, however, a considerable portion of the initial useful layer height of material has to be removed from the grinding pad, for example more than one third. This makes the described method uneconomic (high consumption of expensive grinding pad, high consumption of the trimming blocks, lengthy trimming process with long outage of the installation).